1. Field of the Invention
The present invention relates to a fuel cell having a plurality of electrolyte electrode assemblies interposed between separators. Each of the electrolyte electrode assemblies includes an anode, and a cathode, and an electrolyte interposed between the anode and the cathode.
2. Description of the Related Art
Typically, a solid oxide fuel cell (SOFC) employs an electrolyte of ion-conductive solid oxide such as stabilized zirconia. The electrolyte is interposed between an anode and a cathode to form an electrolyte electrode assembly. The electrolyte electrode assembly is interposed between separators (bipolar plates), and the electrolyte electrode assembly and the separators make up a unit of fuel cell for generating electricity. A predetermined number of fuel cells are stacked together to form a fuel cell stack.
In the fuel cell, an oxygen-containing gas or air is supplied to the cathode. The oxygen in the oxygen-containing gas is ionized at the interface between the cathode and the electrolyte, and the oxygen ions (O2−) move toward the anode through the electrolyte. A fuel gas such as hydrogen-containing gas or CO is supplied to the anode. Oxygen ions react with the hydrogen in the hydrogen-containing gas to produce H2O or react with CO to produce CO2. Electrons released in the reaction flow through an external circuit to the cathode, creating a DC electric current.
Generally, the solid oxide fuel cell is operated at a high temperature in the range from 800° C. to 1000° C. The solid oxide fuel cell utilizes the high temperature waste heat for internal reforming to produce the fuel gas, and generates electricity by spinning a gas turbine. The solid oxide fuel cell is attractive as it has the highest efficiency in generating electricity in comparison with other types of fuel cells, and receiving growing attention for potential use in vehicles in addition to the applications in combination with the gas turbine.
Stabilized zironia has a low ion conductivity. Therefore, the electrolyte membrane formed of stabilized zirconia needs to be thin so that oxygen ions move through the electrolyte membrane smoothly for improving the power generation performance. However, the electrolyte membrane of the stabilized zirconia can not be very thin for maintaining the sufficient mechanical strength. Therefore, it is difficult to produce a large electricity using the membrane of stabilized zirconia in the solid oxide fuel cell.
In an attempt to address the problem, Japanese Laid-Open Patent Publication No. 6-310164 (prior art 1) discloses a solid oxide fuel cell system. In the solid oxide fuel cell system, a plurality of unit cells each having a small surface area are provided on each of metallic separators, and a fuel gas supply hole and an oxygen-containing gas supply hole are formed centrally in each of the unit cells. According to the disclosure of the prior art 1, the fuel cell system has an improved reliability in which the total surface area of the cells on the separator is large, and the substrate is crack-free.
In the prior art 1, the through holes (the fuel gas supply hole and the oxygen-containing gas supply hole) are formed centrally in each of the unit cells. Further, the unit cell has a fuel gas ventilation groove or an oxygen-containing gas ventilation groove. Therefore, the mechanical strength of the unit cell is low. For example, the unit cell is likely to be damaged during the operation of the fuel cell.
Further, Japanese Laid-Open Patent Publication No. 8-279364 (prior art 2) discloses another type of solid oxide fuel cell system. As shown in FIG. 16, each of unit cells 1 is interposed between a pair of separators 2. The unit cell 1 is formed of a thin plate, and does not have any holes. The unit cell 1 and a spacer 3 around the unit cell 1 are interposed between separators 2. The separator 2 has a fuel gas inlet hole 4, an air inlet hole 5 extending in the stacking direction.
The fuel gas from the fuel gas inlet hole 4 flows through a fuel gas distribution passage 6, and is supplied to a central region of one surface of the unit cell 1. The air from the air inlet hole 5 flows through an air distribution passage 7, and is supplied to a central region of the other surface of the unit cell 1.
According to the disclosure, since the unit cell 1 is formed of a thin plate, and does not have any holes, the mechanical strength of the unit cell 1 is high. The reactant gases are supplied outwardly from central regions of opposite surfaces of the unit cell 1 to the reaction areas. Therefore, the two reactant gases are separated from each other.
However, in the prior art 2, the leakage (cross leakage) of the fuel gas from the fuel gas inlet hole 4 may occur. For example, the fuel gas may undesirably reach the cathode of the unit cell 1. Therefore, the local reaction of the air and the fuel gas at the cathode would cause heat generation. Consequently, the unit cell 1 and the separators 2 may be damaged by the heat.